The desired therapeutic response for diathermy is increased muscle blood flow. This reactive hyperemia is a nonlinear, threshold function of temperature. The hyperemia derives from capillary and arteriole opening, thereby increasing pari pasu the muscle blood content. This threshold increase in blood flow/content in the body of the muscle will increase the complex permittivity of the muscle. The change in complex permittivity is detected by a change in the complex impedance at the applicator terminals as the muscle heats through the blood flow threshold. The instrumentation to accomplish this will be a dual channel vector voltmeter with coherent sampling to create very broadband, unity gain mixers. A complex ratio of the test (forward power) and reference (reflected power) channels provides correction for source drift. The magnitude and phase of the complex reflection coefficient will be displayed as functions of time. In addition, an independent arterial flow measurement will be obtained with an FM/CW Doppler system operating at 10 GHz. This data will assist the discrimination of direct temperature effects from the reactive hyperemia. Diathermy will be supplied by a contacting applicator at 918 MHz at a maximum muscle SAR of 150-200 mW/g for 20 min. in normal human volunteers. These noninvasive microwave sensors can be applied to any form of deep heating, but they are especially suitable for microwave diathermy. Thus, they are applicable to existing as well as new diathermy equipment.